Refiners used for manufacturing mechanical pulp typically comprise one or more refiner elements positioned oppositely and rotating relative to each other. The fixed i.e. stationary refiner element is called the stator of the refiner, and the rotating or rotatable refiner element is called the rotor of the refiner. In disc refiners, the refiner elements are disc-like and in cone refiners the refiner elements are conical. In addition to disc refiners and cone refiners, there are also what are called disc-cone refiners where disc-like refiner elements come first in the flow direction of the material to be defibrated and after them the material to be defibrated is refined further between conical refiner elements. Furthermore, there are also cylindrical refiners where both the stator and the rotor of the refiner are cylindrical refiner elements.
The refining surfaces of the refiner elements are formed by bars, i.e. bars and blade grooves i.e. grooves between the bars. The task of the bars is to defibrate the lignocellulosic material and the task of the grooves is to transport both material to be defibrated and material already defibrated on the refining surface. In disc refiners, which represent the most common refiner type, the material to be refined is usually fed through an opening in the middle of the stator i.e. on the inner periphery of the refining surface of the stator, to the space between the refiner surfaces of the discs i.e. to a blade gap. The refined material is discharged from the blade gap, from the outer periphery of the refining surfaces of the refiner discs, to be fed onwards in the pulp manufacturing process. The refining surfaces of the refiner discs may be either surfaces formed directly on the refiner discs, or they may be formed as separate blade segments positioned adjacent to each other in such a way that each blade segment forms part of a continuous refining surface. The same is true for cone refiners as well.
Usually, dams connecting two adjacent bars to each other are positioned at the bottom of the blade grooves of the refining surfaces of both the stator and the rotor of the refiner. The task of the dams is to guide the material to be refined and material already refined to the space between the bars of opposite refining surfaces to be further refined. Since the dams guide the material to be refined to the space between opposite blade bars, refining the material can be promoted thanks to the dams. Simultaneously, however, the dams cause the steam flow taking the material to be refined onwards in the blade grooves to decrease and prevent passage of the material to be refined and the material already refined on the refining surface by restricting the cross-sectional flow area of the blade grooves. This in turn leads to blockage on the refining surface, which then results in a decrease in the production capacity of the refiner, non-uniformity of the quality of the refined material and an increase in the energy consumed for the refining.
WO 2010/106225 A1 describes a refining surface that does not use dams for guiding the material into the blade gap between the opposite refining surfaces. The refining surface comprises first and second blade bars with blade grooves between them, as well as third blade bars located in the blade grooves between the first and second blade bars. The third blade bars have sloping ends that ascend from the bottom of the blade grooves up to the upper surfaces of the blade bars. The sloping ends are located at the end of the blade bars closest to the feed edge of the refining surface and thus form rising guide surfaces for guiding the material from the blade grooves between the blade bars to the upper surfaces of the blade bars and into the blade gap.
In any continuous process, minimizing variations is crucial for maximizing quality, minimizing costs and getting a stable process. This is also true for any pulp refining process in which fiber (wood or other lignocellulosic material.) is refined between refiner segments. The term lignocellulose refers to plant dry matter or so called lignocellulosic biomass. It is composed of carbohydrate polymers (e.g. cellulose, hemicellulose), and an aromatic polymer (lignin). These carbohydrate polymers contain different sugar monomers (six and five carbon sugars) and they are tightly bound to lignin. Lignocellulosic biomass can be broadly classified into virgin biomass, waste biomass and energy crops. Virgin biomass includes all naturally occurring terrestrial plants such as trees, bushes and grass. Waste biomass is produced as a low value byproduct of various industrial sectors such as agricultural (corn stover, sugarcane bagasse, straw etc), forestry (saw mill and paper mill discards). Energy crops are crops with high yield of lignocellulosic biomass produced to serve as a raw material for production of second generation biofuel examples include switch grass (Panicum virgatum) and Elephant grass.
Within pulp refining, variations in feed within the refining gap between the stator and rotor segments causes an increase in energy needed to maintain a predetermined or desired pulp quality and causes variations in end fiber quality. Therefore, there is a need for improving the design of the blade segments in order to overcome the above mentioned disadvantages.